The present invention relates to a process for producing molten metal (which term includes metal alloys), in particular although by no means exclusively iron, from metalliferous feed material, such as ores, partially reduced ores and metal-containing waste streams, in a metallurgical vessel containing a molten bath.
The present invention relates particularly to a molten metal bath-based direct smelting process for producing molten metal from a metalliferous feed material.
The most widely used process for producing molten iron is based on the use of a blast furnace. Solid material is charged into the top of the furnace and molten iron is tapped from the hearth. The solid material includes iron ore (in sinter, lump or pellet form), coke, and fluxes and forms a permeable burden that moves downwardly. Preheated air, which may be oxygen enriched, is injected into the bottom of the furnace and moves upwardly through the permeable bed and generates carbon monoxide and heat by combustion of coke. The result of these reactions is to produce molten iron and slag.
A process that produces iron by reduction of iron ore below the melting point of the iron produced is generally classified as a xe2x80x9cdirect reduction processxe2x80x9d and the product is referred to as DRI.
The FIOR (Fluid Iron Ore Reduction) process is an example of direct reduction process. The process reduces iron ore fines as the fines are gravity-fed through each reactor in a series of fluid bed reactors. The fines are reduced in solid state by compressed reducing gas that enters the bottom of the lowest reactor in the series and flows counter-current to the downward movement of fines.
Other direct reduction processes include moving shaft furnace-based processes, static shaft furnace-based processes, rotary hearth-based processes, rotary kiln-based processes, and retort-based processes.
The COREX process includes a direct reduction process as one stage. The COREX process produces molten iron directly from coal without the blast furnace requirement of coke. The COREX process includes 2-stage operation in which:
(a) DRI is produced in a shaft furnace from a permeable bed of iron ore (in lump or pellet form) and fluxes; and
(b) the DRI is then charged without cooling into a connected melter gasifier and melted.
Partial combustion of coal in the fluidised bed of the melter gasifier produces reducing gas for the shaft furnace.
Another known group of processes for producing iron is based on cyclone converters in which iron ore is melted by combustion of oxygen and reducing gas in an upper melting cyclone and is smelted in a lower smelter containing a bath of molten iron. The lower smelter generates the reducing gas for the upper melting cyclone.
A process that produces molten metal directly from ores (and partially reduced ores) is generally referred to as a xe2x80x9cdirect smelting processxe2x80x9d.
One known group of direct smelting processes is based on the use of electric furnaces as the major source of energy for the smelting reactions.
Another known direct smelting process, which is generally referred to as the Romelt process, is based on the use of a large volume, highly agitated slag bath as the medium for smelting top-charged metal oxides to metal and for post-combusting gaseous reaction products and transferring the heat as required to continue smelting metal oxides. The Romelt process includes injection of oxygen enriched air or oxygen into the slag via a lower row of tuyeres to provide slag agitation and injection of oxygen into the slag via an upper row of tuyeres to promote post-combustion in the Romelt process the metal layer is not an important reaction medium.
Another known group of direct smelting processes that are slag-based is generally described as xe2x80x9cdeep slagxe2x80x9d processes. These processes, such as DIOS and AISI processes, are based on forming a deep layer of slag. As with the Romelt process, the metal layer below the slag layer is not an important reaction medium.
Another known direct smelting process which relies on a molten metal layer as a reaction medium, and is generally referred to as the HIsmelt process, is described in International application PCT/AU96/00197 (WO 96/31627) in the name of the applicant.
The HIsmelt process as described in the International application comprises:
(a) forming a molten bath having a metal layer and a slag layer on the metal layer in a vessel;
(b) injecting into the bath:
(i) a metalliferous feed material, typically metal oxides; and
(ii) a solid carbonaceous material, typically coal, which acts as a reductant of the metal oxides and a source of energy; and,
(c) smelting the metalliferous feed material to metal in the metal layer.
The HIsmelt process also comprises post-combusting reaction gases, such as CO and H2, released from the bath in the apace above the bath with oxygen-containing gas and transferring the beat generated by the post-combustion to the bath to contribute to the thermal energy required to smelt the metalliferous feed materials.
The HIsmelt process also comprises forming a transition zone above the nominal quiescent surface of the bath in which there are ascending and thereafter descending droplets or splashes or streams of molten metal and slag which provide an effective medium to transfer to the bath the thermal energy generated by post-combusting reaction gases above the bath.
A preferred form of the HIsmelt process is characterized by forming the transition zone by injecting carrier gas, metalliferous feed material, solid carbonaceous material and optionally fluxes into the bath through lances that extend downwardly and inwardly through side walls of the vessel so that the carrier gas and the solid material penetrate the metal layer and cause molten material to be projected from the bath.
This form of the HIsmelt process is an improvement over earlier forms of the process which form the transition zone by bottom injection of carrier gas and solid carbonaceous material through tuyeres into the bath which causes droplets, splashes and streams of molten material to be projected from the bath.
The applicant has carried out extensive pilot plant work on the above-described preferred form of the HIsmelt process and has made a series of significant findings in relation to the process.
One of the findings made by the applicant, which forms the basis of the present invention, is that the upward flow rate of bath-derived gas caused by the injection of solid material/carrier gas into the molten bath should be at least 0.30 Nm3/s/m2 at the location of the interface of the metal layer and the slag layer (under quiescent conditions) to establish the transition zone so that there is heat transfer to the molten bath at an effective rate.
Heat transfer efficiency is a measure of the amount of the available energy generated by post combustion that is transferred to the molten bath. It is also a measure of the amount of the available energy generated by post combustion that is lost from the vessel (via discharge of off-gas above bath temperature and heat transfer via the side walls and roof of the vessel).
The minimum bath-derived gas flow rate of 0.30 Nm3/s/m2 at the interface of the metal layer and the slag layer (under quiescent conditions) ensures that there is sufficient buoyancy uplift of splashes, droplets and streams of molten material from the molten bath into the transition zone to maximise:
(a) heat transfer to the molten bath via subsequently descending splashes, droplets and streams of molten material; and
(b) contact of molten material with the side walls of the vessel which forms a protective layer of slag that reduces heat loss from the vessel.
Item (b) above is a particularly important consideration in the context of the preferred vessel construction of the present invention which includes water cooled panels that form the side walls in the upper barrel section and optionally the roof and water cooled refractory bricks that Form the side walls in the lower barrel section of the vessel.
In general terms, the present invention is a direct smelting process for producing metal from a metalliferous feed material in a fixed, ie not-rotatable, metallurgical vessel, which process includes the steps of:
(a) forming a molten bath having a metal layer and a slag layer on the metal layer in the vessel;
(b) injecting metalliferous feed material and/or solid carbonaceous material with a carrier gas into the molten bath via one or more than one downwardly extending lance/tuyere and smelting metalliferous material in the molten bath, whereby the solids and gas injection causes gas flow from the molten bath at a flow rate of at least 0.30 Nm3/s/m2 at the location of the interface between the metal layer and the slag layer (under quiescent conditions), which gas flow entrains molten material in the molten bath and carries molten material upwardly as splashes, droplets and streams and forms a transition zone in a gas continuous space in the vessel above the slag layer, whereby splashes, droplets and streams of molten material contact the side walls of the vessel and form a protective layer of slag; and
(c) injecting an oxygen-containing gas into the vessel via one or more than one lance/tuyere and post-combusting reaction gases released from the molten bath, whereby ascending and thereafter descending splashes, droplets and streams of molten material facilitate heat transfer to the molten bath, and whereby the transition zone minimises radiation heat loss from the vessel via the side walls in contact with the transition zone.
The above-described gas flow rate of at least 0.30 Nm3/s/m2 at the location of the interface of the metal layer and the slag layer (under quiescent, conditions) is a substantially higher bath-derived gas flow rate than the Romelt process and the deep-slag process such as the DIOS and AISI processes described above and is a significant difference between the process of the present invention and these known direct smelting processes.
By way of particular comparison, U.S. Pat. No. 5,078,785 of Ibaraki et al (assigned to Nippon Steel Corporation) discloses a particular form of a deep-slag process using a rotatable vessel and discloses bottom injection of gas into a metal layer for the purpose of metal bath agitation. The paragraph commencing at line 17 of column 14 discloses that it is preferred that the xe2x80x9cmetal bath agitation forcexe2x80x9d generated by the bottom gas injection be no more than 6 kW/t. The U.S. patent discloses that at higher levels of agitation there may be undesirably high levels of iron dust generation. On the basis of the information provided in the paragraph commencing at line 21 of column 14, a maximum metal bath agitation force of 6 kw/t corresponds to a maximum bath-derived gas flow rate of 0.12 Nm3/s/m2 at the interface between the metal layer and the slag layer. This maximum gas flow rate is considerably below the minimum flow rate of 0.30 Nm3/s/m2 of the present invention.
Preferably the process includes smelting metalliferous material to metal mainly in the metal layer.
Preferably the solids and gas injection in step (b) causes gas flow from the molten bath substantially across the interface between the metal layer and the metal slag layer (under quiescent conditions).
Preferably the gas flow rate is at least 0.35 Nm3/s/m2, more preferably at least 0.50 Nm3/s/m2, at the location of the interface between the metal layer and the slag layer (under quiescent conditions).
Preferably the gas flow rate is less than 0.90 Nm3/s/m2 at the location of the interface between the metal layer and the slag layer (under quiescent conditions).
Typically, the splashes, droplets and streams of molten material entrain further molten material (particularly slag) as they move upwardly.
Typically, slag is a major part and molten metal is the remaining part of the molten material in the splashes, droplets and streams of molten material.
The term xe2x80x9csmeltingxe2x80x9d is understood herein to mean thermal processing wherein chemical reactions that reduce metal oxides take place to produce liquid metal.
The term xe2x80x9cmetal layerxe2x80x9d is understood herein to mean that region of the bath that is predominantly metal. Specifically, the term covers a region or zone that includes a dispersion of molten slag in a metal continuous volume.
The term xe2x80x9cslag layerxe2x80x9d is understood herein to mean that region of the bath that is predominantly slag. Specifically, the term covers a region or zone that includes a dispersion of molten metal in a slag continuous volume.
Preferably the transition zone extends above the slag layer
It is preferred that the level of dissolved carbon in metal be greater than 4 wt %.
It is preferred that the concentration of FeO in the slag layer be below 5 wt %.
It is preferred that the process further comprises selecting the amount of the solid carbonaceous material injected into the molten bath to be greater than that required for smelting the metalliferous feed and for generating heat to maintain reaction rates such that dust entrained in off-gas leaving the vessel contains at least some excess carbon.
It is preferred that the concentration of solid carbon in dust in off-gas from the vessel be in the range of 5 to 90 wt % (more preferably 20 to 50 wt %) of the weight of dust in the off-gas at a rate of dust generation of 10-50 g/Nm3 in the off-gas.
The injection of metalliferous material and carbonaceous material may be through the same lance/tuyere or separate lances/tuyeres.
The transition zone is quite different to the slag layer. By way of explanation, under stable operating conditions of the process the slag layer comprises gas bubbles in a liquid continuous volume whereas the transition zone comprises splashes, droplets, and streams of molten material, predominantly slag, in a gas continuous volume.
Preferably step (c) of the process post-combusts reaction gases, such as carbon monoxide and hydrogen, generated in the molten bath, in a top space (including the transition zone) above the surface of the molten bath and transfers the heat generated by the post-combustion to the molten bath to maintain the temperature of the molten bathxe2x80x94as is essential in view of endothermic reactions in the molten bath.
Preferably the one or more than one oxygen-containing gas injection lance/tuyere is positioned to inject the oxygen-containing gas into a central region of the vessel.
The oxygen-containing gas may be oxygen, air or oxygen enriched air containing up to 40% oxygen by volume.
Preferably the oxygen-containing as is air.
More preferably the air is pre-heated.
Typically, the air is preheated to 1200xc2x0 C.
The air may be oxygen enriched.
Preferably step (c) of the process operates at high levels, ie at least 40%, of post-combustion, where post-combustion is defined as:             [              CO        2            ]        +          [                        H          2                ⁢        O            ]                  [              CO        2            ]        +          [                        H          2                ⁢        O            ]        +          [      CO      ]        +          [              H        2            ]      
where:
[CO2]=volume % of CO2 in off-gas;
[H2O]=volume % of H2O in off-gas;
[CO]=volume % of CO in off-gas; and
[H3]=volume % of H2 in off-gas.
In some instances a supplementary source of solid or gaseous carbonaceous material (such as coal or natural gas) may be injected into the off-gas from the vessel in order to capture thermal energy in the form of chemical energy.
An example of such supplementary injection of carbonaceous material is injection of natural gas which cracks and reforms, and thus cools, the off-gas whilst enriching its fuel value.
The supplementary carbonaceous material may be added in the upper reaches of the vessel or in the off-gas duct after the off-gas has left the vessel.
Preferably the process operates at a post-combustion greater than 50%, more preferably greater than 60%.
Preferably, the one or more than one lance/tuyere extend through the side walls of the vessel and are angled downwardly and inwardly towards the metal layer.
Preferably the location and operating parameters of the one or more than one lance/tuyere that injects the oxygen-containing gas and the operating parameters that control the transition zone are selected so that:
(a) the oxygen-containing gas is injected towards and penetrates the transition zone;
(b) the transition zone extends upwardly around the lower section of the or each lance/tuyere and thereby shields to some degree the side walls of the vessel from the combustion zone generated at the end of the or each lance/tuyere; and
(c) there is gas continuous space described as a xe2x80x9cfree spacexe2x80x9d which contains practically no metal and slag around the lend of the or each lance/tuyere.
Item (c) above is an important feature because it makes it possible for reaction gases in the top space of the vessel to be drawn into the region at the end of the or each lance/tuyere and be post-combusted in the region.
Preferably the process maintains a relatively high (but not too high) slag inventory and uses the amount of slag as a means of controlling the process.
The term xe2x80x9crelatively high slag inventoryxe2x80x9d may be understood in the context of the amount of slag compared to the amount of metal in the vessel.
Preferably, when the process is operating under stable conditions, the weight ratio of metal:slag is between 4:1 and 1:2.
More preferably the weight ratio of metal:slag is between 3.1 and 1:1.
It is preferred particularly that the metal:slag weight ratio be between 2:1 and 1:1.
The term relatively high slag inventory may also be understood in the context of the depth of slag in the vessel.
Preferably the process includes maintaining the high slag inventory by controlling the slag layer to be 0.5 to 4 metres deep under stable operating conditions.
More preferably the process includes maintaining the high slag inventory by controlling the slag layer to be 1.5 to 2.5 metres deep under stable operating conditions.
It is preferred particularly that the process includes maintaining the high slag inventory by controlling the slag layer to be at least 1.5 metres deep under stable operating conditions.
The amount of slag in the slag layer of the molten bath has a direct impact on the amount of slag that is in the slag-rich transition zone.
The slag is important in the context of minimising heat loss via radiation from the transition zone to the side walls of the vessel.
If the slag inventory is too low there will be increased exposure of metal in the slag-rich transition zone ark therefore increased oxidation of metal and the potential for reduced post-combustion.
If the slag inventory is too high then the one or more than one oxygen-containing gas injection lance/tuyere become buried in the transition zone and this minimises movement of top space reaction gases to the end of the or each lance/tuyere and, as a consequence, reduces potential for post-combustion.
According to the present invention there is provided a fixed, ie non-rotatable, vessel which produces metal from a metalliferous feed material by a direct smelting process, which vessel contains a molten bath having a metal layer and a slag layer on the metal layer and has a gas continuous space above the slag layer, which vessel includes:
(a) a shell;
(b) a hearth formed of refractory material having a base and sides in contact with the molten bath;
(c) side walls which extend upwardly from the sides of the hearth and are in contact with the slag layer and the gas continuous space, wherein the side walls that contact the gas continuous space include water cooled panels and a layer of slag on the panels;
(d) one or more than one lance/tuyere extending downwardly into the vessel and injecting an oxygen-containing gas into the vessel above the metal layer;
(e) one or more than one downwardly and inwardly extending lance/tuyere injecting at least part of the metalliferous feed material and/or a carbonaceous material with a carrier gas into the molten bath so as to generate bath-derived gas flow at a rate of at least 0.30 Nm3/s/m2 at the location of the interface between the metal layer and the slag layer (under quiescent conditions) and resultant buoyancy uplift of molten material from the metal layer and the slag layer;
(f) a transition zone formed by ascending and thereafter descending splashes, droplets and streams of molten material in the gas continuous space above the slag layer with some of these splashes, droplets and streams contacting the side walls of the vessel and forming a layer of molten material on the side walls; and
(g) a means for tapping molten metal and slag from the vessel.
Preferably the solids and gas injection via the lance/tuyere or lances/tuyeres generates gas flow from the molten bath substantially across the interface between the metal layer and the slag layer (under quiescent conditions).
Preferably the vessel includes a cylindrical hearth and aide walls which form a cylindrical barrel extending from the hearth.
The metalliferous feed material may be any suitable material and in any suitable form. A preferred metalliferous feed material is an iron containing material. The iron-containing material may be in the form of ores, partially reduced ores, DRI (direct reduced iron), iron carbide, millscale, blast furnace dust, sinter fines, BOF dust or a mixture of such materials.
In the case of partially reduced ores, the degree of pre-reduction may range from relatively low levels (eg to FeO) to relatively high levels (eg 70 to 95% metallisation).
In this connection, the process further includes partially reducing metalliferous ores and, thereafter injecting the partially reduced ores into the molten bath.
The metalliferous feed material may be pre-heated.
The carrier gas may be any suitable carrier gas.
It is preferred that the carrier gas be an oxygen-deficient gas.
It is preferred that the carrier gas comprise nitrogen.